Process of acoustic emission for sonar

ABSTRACT

A process of acoustic emission for sonar with a separate emission array from the receiving array. The emission array has the shape of a linear acoustic array and can either be towed simultaneously with linear acoustic receiving array or be suspended from a helicopter to form a &#34;dipping&#34; type sonar. Detection using this type of sonar is facilitated by increasing the sound level via the directivity index, while reducing the level of reverberation originating from the bed and from the surface of the sea.

The present invention relates to processes which make it possible toemit acoustic signals in water. More particularly, it concerns theapplication of such a process to systems for activating towed lineararrays. It also concerns the application of this process to deployableactive sonar systems for aircraft known as "dipping" sonars.

DISCUSSION OF THE BACKGROUND

It is known that sonars are conventionally divided into two majorcategories. One of these categories comprises active sonars in which anacoustic signal is emitted within the sea and the echoes returned by theobstacles whose position one wishes to ascertain are gathered. The othercategory comprises passive sonars which are concerned merely withlistening for the noise radiated within the sea by various sources, forexample the propellers of a boat. Such is the case in particular forlinear acoustic arrays which are towed behind the boat and which stretchover distances of several hundred meters.

This technique has been employed to design devices which enable thesearrays, essentially characteristic of a passive sonar, to be made toplay a role of active sonar, by using a distinct special-purpose arrayto emit an acoustic signal whose echoes are received by these arrays.This function is termed the "activation function".

Referring to FIG. 1, the linear acoustic array 101 is towed by a boat102, and in order to use it in an active manner this same boat 102 towsa fish 103 which comprises an acoustic emitter. Given the ranges offrequencies used by a linear acoustic array, which lie within thelow-frequency range, the emitter contained in the fish emits in anear-omnidirectional manner by virtue of the small size of its emissionarrays relative to the emission wavelength λ, made necessary by thedimensions of the fish. Such a device is in particular described inFrench Patent Application No. 91 03853 filed by the applicant on 29 Mar.1991 and published on 2 Oct. 1992 under number 2 674 717.

There is also a type of active sonar, known as a "dipping" sonar andrepresented in FIG. 2, which includes receiving arrays 201 carried byfolding arms 211 fixed to a submersible buoy body 221 and which aredeployed when the body of the buoy is submerged at the required depth.This body is suspended by a cable wound on a winch fixed to a helicopter202. It also includes an emitter 203 situated for example at the base ofthe buoy. The helicopter, which hovers at a fixed spot, unwinds thecable, and the body which carries the receiving array is submerged inthe water. Such a sonar is in particular described in French PatentApplication No. 86 13485 filed by the applicant on 26 Sep. 1986 andpublished on 1 Apr. 1988 under number 2 604 530. Although thefrequencies used are often higher than the frequencies used in thelinear array of FIG. 1, the small size of the body of the buoy precludesthe fixing of a very large emitter and so the same relative conditionsas before again obtain, involving substantially omnidirectionalradiation of the emitter.

As a consequence of the omnidirectional nature of the radiation, thesedevices have the drawback of limiting the power emitted, so as not torun up against the phenomenon of cavitation, this limiting the detectionrange since the sound level emitted under these conditions is relativelylow and since the directivity index cannot be altered precisely becauseof this omnidirectional nature.

Moreover, since we are at low frequency and because this antenna isrelatively small, nuisance acoustic interactions are obtained betweenthe various transducers, making control of the directivity patterns moredifficult.

Finally, and more particularly in the case of a linear acoustic array,the handling of a fish, even if it is relatively small in size, involvesimplementational constraints related to weight and bulk, which arehighly constraining during placement into the water and recovery, andhence entailing a very high cost.

SUMMARY OF THE INVENTION

To alleviate these drawbacks, the invention proposes a process ofacoustic emission for sonar, principally characterized in that aseparate linear acoustic array is used to emit acoustic signals whoseechoes are received by the receiving array of the sonar.

In order to implement this process, the invention furthermore proposes adevice of the type comprising a passive linear acoustic array intendedto be towed by a boat with the aid of a hauling cable, characterizedessentially in that a linear acoustic emission array, whose weight andshape enable it to be towed at a small inclination relative to thevertical, is fixed to the hauling cable, whilst the receiving array istowed substantially horizontally.

According to another characteristic, this device is of the typecomprising a sonar with deployable receiving array intended to besuspended from a helicopter by a cable, and it furthermore comprises alinear acoustic emission array (803) intended to be suspended above orbeneath this receiving array.

According to another characteristic, the cable for suspending the sonarforms an integral part of the linear emission array.

According to another characteristic, the linear acoustic emission arrayis formed by a string of transducers fixed along at least one cable.

According to another characteristic, the transducers are of the flexuralstrain gauge type.

According to another characteristic, the device includes means forsupplying the transducers with variably weighted signals for controllingthe shape of the main emission lobe.

According to another characteristic, the device furthermore comprisesmeans for supplying the transducers with phase-shifted signals making itpossible to shift the aim of the emission beam.

According to another characteristic, the device furthermore comprises afish dragged by the towing cable and itself towing the linear receivingarray; this fish including an integrated winch to which the linearemission array is fastened so as to enable this array to be wound inbefore the fish and the linear receiving array are together raised ontothe hauling vessel.

According to another characteristic, the linear emission array is formedby a set of identical modules comprising a transducer of the flexuralstrain gauge type with substantially ovoid cross-section furnished witha rear fairing of substantially triangular cross-section and including,at its two ends, shackles of which one is single whilst the other isdouble so as to enable the single shackle of one module to be insertedinto the double shackle of the adjacent module and to join them togetherwith a fixing pin.

According to another characteristic, the transducers are articulatedtogether so as to form a winch-wound assembly.

According to another characteristic, the emission array comprises meansfor mechanically interlocking these articulations so as to ensure thestraightness of the array during emission and so as to release thislocking during the operations of winding and unwinding the array on awinch.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will emerge clearly inthe following description given by way of non-limiting example withregard to the appended figures which represent:

FIGS. 1 and 2, two passive sonars according to the prior art;

FIG. 3, a passive sonar activated according to the invention;

FIGS. 4 and 5, two variant embodiments of the emission transducers of asonar according to the invention;

FIGS. 6 and 7, two variants of a supply system for these transducers;

FIG. 8, a variant embodiment relating to a sonar of the "dipping" type;

FIG. 9, a device for winding the emitting array of a sonar according tothe invention; and

FIG. 10, end and side views of a link of an emitting array according tothe invention.

DISCUSSION OF THE PREFERRED EMBODIMENT

In the embodiment of the invention represented in FIG. 3, a towboat 302hauls a passive linear array 301 with the aid of a hauling cable 305.This passive linear array is of known type and a description thereofwill be found for example in French Patent Application No. 90 15474filed by the applicant on 11 Dec. 1990 and published on 12 Jun. 1992under number 2 670 350.

This purely passive array therefore receives all the noise, especiallylow frequency noise, propagating within the sea, for example the noisefrom the propellers of boats.

To use it in an active manner, so that it receives the echoes of a sonaremission arising from a system into which it is integrated, use is madeaccording to the invention of a linear acoustic emission array 303,itself fastened to the cable 305 so as to be towed by the boat 302. Thislinear emission array is formed by a string of transducers, preferablyof the flexural strain gauge type, advantageously fixed along one ormore cables and placed in a flexible jacket. In order to hold thislinear array 303 as vertical as possible despite the drag arising fromthe hauling, the latter preferably terminates in a nonactive part and/ora dipper 304 which enables it to be sufficiently ballasted to obtain aposition close to the desired vertical position.

Different types of transducers may be envisaged, but the transducers ofthe flexural strain gauge type are particularly suitable for this use,and a diagrammatic view of a cross-section of the array 303 including asingle one of these transducers is represented in FIG. 4. Thistransducer is a flexural strain gauge of the type known as a class IVcomposed essentially of an elliptic shell 401 which includes apiezoelectric motor 402 along its major axis. In order to be able toform a string intended to make the array 303, the two ends of theelliptic shell are drilled longitudinally to make it possible to runcables 403 onto which the successive transducers will be threaded.Fixing means, not represented, make it possible to lock thesetransducers at predetermined positions on the cable and to ensure thestraightness of the array in the operational position so as to obtainthe requisite directivity pattern in emission. The transducers are wiredup with the aid of connection cables, not represented, which stretchsubstantially parallel to the structural cables 403. The assembly isplaced inside a flexible jacket, itself filled with a protective liquidaccording to the same technique as that used for the linear arrays suchas the array 301. The density of the liquid will preferably be greaterthan that of water so as to increase the weight of the array and limitits inclination under the drag forces.

It is possible to use other types of transducers able to emit acousticwaves at the desired frequency and with the requisite power, so long asthey can be manufactured at sufficiently small dimensions. By way ofexample, represented in FIG. 5 is another embodiment which uses aflexural strain gauge transducer of the type known as a "barrel stave",which is symmetric in the axis of the array. This transducer comprises abobbin-shaped shell 501 which encloses a cylinder-shaped axialpiezoelectric motor 502. The assembly is drilled with an axial holewhich makes it possible to run a single cable 503, which plays the samerole as the two cables 403 of the example represented in FIG. 4.

Other structures could also be used, for example devices comprisingarticulated or flexible universal joints enabling the transducers to beinterlinked.

Since the emission array is required to navigate as vertically aspossible, it is beneficial for its apparent density with respect towater to be as large as possible. Given the use of a large number oftransducers which are themselves fairly heavy, the initial density ofthe structure is already relatively high. The latter will be increasedby using the densest possible filling liquid, possibly loading it with agranular material which is itself as dense as possible. The weight ofthe array, which enables it to be held as vertical as possible,furthermore allows it to be made to act as a sinker, a role whichgenerally devolves upon particular apparatuses, which generally take theform of an inverted aircraft wing which, by pulling the towing cable 305towards the bed, enables the passive receiving array 301 to be held at aspecified submersion while also holding it horizontally in as straight amanner as possible.

It is desirable for the linear emission array 303 to itself be asstraight as possible, and for this purpose it is preferably accorded awater droplet profile, the axis of which is directed in the direction ofhauling so as to reduce the drag coefficient. The use of IV typetransducers such as represented in FIG. 4 implies an alreadysubstantially streamlined shape at the outset. In order perhaps toimprove this, or should the basic structure be poorly streamlined, asfor example in the embodiment of FIG. 5, it will be advantageous to usea fairing of appropriate shape. Furthermore, to limit the occurrence ofvibrations at operational towing speeds, it will be beneficial to extendthe active part of the array 303 with an inactive part 304 of the samedensity and the same cross-section as that of the active part.

Since the drag of the assembly is fairly large, it is beneficial whenthey are not being used to raise the linear arrays onto the deck of thetowboat and to stow them in a minimum of space. This can be done byusing a winch 306 which will make it possible to wind up the two arrays,preferably onto two separate drums each including a cable-guides device.

Even with such a towing system it is still not possible to hold theemission array 303 strictly vertical, and the latter is thereforeinclined rearwards of the direction of advance by an angle θ_(c)relative to the vertical.

Calling W the apparent weight per linear meter in the water of the array303, P the density of water, S the end cross-section of this array, Cxits drag coefficient, and V the towing speed, the total drag on thearray is given by the formula: ##EQU1##

The angle of inclination θ_(c) relative to the vertical is then given bythe formula: ##EQU2##

By way of example, for a towing speed of 8 knots, a cross-section of 0.1m² corresponding to a diameter of 10 cm, and a relative density of thearray equal to 5, the angle of inclination θ_(c) is equal to 10°. Thisamounts to saying that the linear array navigates at a slant of 10°relative to the vertical.

It is known that for a linear array comprising N transducers equispacedby λ/2, the directivity index is given by 10 log N. Under theseconditions if a gain of for example 15 dB is required, 30 transducersshould be used, this demanding a linear array whose length is equal to15 λ. If the frequency used is for example 1 kHz, this length is thenequal to 22.50 m. It is observed that for the same power it is possibleaccording to the invention to use the directivity index to increaseperformance.

If these transducers are supplied in parallel without special weighting,the width of the main lobe of the channel transverse to the direction ofthe array, referred to as "broadside", is substantially equal to λ/L inradians, i.e. in the above example 4°.

It is also known that in the case of a linear array the most slenderchannels are those which are formed broadside on to the array, both inemission and in reception. The best angular resolution in bearing forthe linear receiving array 301 is therefore obtained on each side of thelatter, perpendicularly to its axis.

For its part the linear emission array emits, if the transducers areactually supplied in parallel in a plane perpendicular to its directionand with radial symmetry about this direction. The inclination of thebeam relative to the horizontal is therefore equal to +θ_(c) rearward ofthe motion of the array and to -θ_(c) forward, given the inclinationdescribed above. By contrast, on the sides the inclination is zero andthe marine volume is swept with sound on these sides with no angularshift relative to the bed and to the surface. Since the receiving arrayis provided essentially to receive on the sides, the sound-sweepcoincides with the direction of listening and in these cases there istherefore no need to shift the aim of the emission channels, which couldbe achieved for example by supplying the various transducers out ofphase.

Represented in FIG. 6 is an example of a circuit for supplying thevarious transducers 601 making up the emission array, in the case inwhich it is desired to weight the supply level for these transducers soas to obtain, in a known manner, a more slender main lobe and attenuatedsidelobes. The emission signal, of the form sin ω₀ t, arrives via apower bus 602. This bus is connected to the various transducers 601 byway of tapped transformers 603, themselves linked to switches 604. Theseswitches 604 are of the digitally programmable type and receiveprogramming signals by way of a bus 605 for controlling amplitudes.These signals thus make it possible to switch the taps of thetransformers 603, in such a way that signals whose amplitude A_(i)depends on the transducer and has been calculated, according to a knownmethod, to obtain the desired weighting can be applied to thetransducers 601.

It may nevertheless be desired to be able to shift the aim of theemission beams in such a way as to have accurate sound-sweeping in asector lying outside the transverse plane, as defined above, of thereceiving array. This makes it possible for example to be able toisolate and track a previously pinpointed echo which appears tooriginate from a sector lying outside the transverse sectors.

To do this, a system such as that represented in FIG. 7 will for examplebe used, in which a power supply bus 702 makes it possible to distributea set of power signals shifted in phase over 360° along the lineararray. In the example represented, there is a base signal sin ω₀ t andtwo other signals shifted respectively by 2π/3 and 4π/3. According to avariant, the base signal is used and 3 signals offset respectively byπ/2, -π/2 and π are added thereto. These signals are applied to twotapped transformers 703 and 713 by way of a switch 704. The taps of thistransformer are themselves applied to switches 714 and 724 so as to beable to supply the transducers 701 differentially. Digital controlsignals are sent to the switches 704, 714 and 724 by a control bus 705,and these make it possible using the switch 704 to select the correctlyphase-shifted supply bus line and using the switches 714 and 724 theamplitude levels required for the weighting.

The switch 714 therefore delivers a signal sinφicosω₀ t and the switch724 a signal cosφisinω₀ t. The combining of these two signals in thetransducer 701 makes it possible to obtain an acoustic output signalA_(i) sin(ω₀ t+φi).

The use of directive emission makes it possible, as well as increasingthe directivity index, to increase the echo/reverberation ratio, thisbeing all the more useful the smaller the depth. Indeed, in this casethe reverberation arising from elevational directions lying outside themain emission lobe is greatly attenuated, this being paramount inrespect of the signal-to-noise ratio when operating in shallow waters.

In order to cover the whole space it is furthermore beneficialalternately to form a channel shifted in elevation by +θ_(c) and achannel offset in elevation by -θ_(c).

In the first case, the whole of the rear space is swept horizontallywith sound without obtaining any reflection on the surface and on thebed, whereas the forward beam is aimed towards the bed at an angle ofelevation equal to -2θ_(c), this increasing the reverberation, which isadvantageously eliminated at processing level, for example by windowing(elimination of close echoes).

In the second case, the reverse effect is obtained and the whole of theforward space is swept with sound horizontally, whilst the rear beam isaimed at the surface with an elevation equal to +2θ_(c), here again withthe reverberation being eliminated through the processing.

To obtain these two successive shifts of aim, it is for example possibleto use the device represented in FIG. 7 by supplementing it with aswitch suitably programmed on the one hand according to two phase lawsand on the other hand so as to transfer from one law to the other.

It is also possible, for a shift in aim with given elevation, to varythe width of the main lobe in a programmable manner by simultaneouslymodifying the amplitudes and phases of the signals applied to thetransducers. To do this, it is for example possible slightly to modifythe phase φ_(i) used, adding a further phase shift +φ_(i) thereto, insuch a way that a phase law which is symmetric with respect to thedirection of aim is applied to the transducers.

By way of enhancement, the invention proposes also to furnish the linearemission array 303 with a set of receiving hydrophones so as to be ableto form reception channels in elevation. The submersion of the targetcan thus be measured instantaneously, either by performing a directmeasurement or by performing appropriate processing, for example of themono-pulse type.

The invention is not limited to the case of the acoustic linear arraytowed as in FIG. 3, but extends also to the case of the dipping sonarsuspended from a helicopter, as represented in FIG. 8.

In this case the helicopter 802 supports the dipping sonar 821 with theaid of a cable 805.

According to the invention, a linear acoustic emission array 803, whichplays the same role as the antenna 303 of FIG. 3, has been fixed to thiscable. Since the helicopter remains stationary as the sonar is loweredwhilst hovering, there is no longer any problem of inclination or anyproblem of drag. The embodiments of this array 803 will be able to besimilar to all those used for the embodiment of the array 303, but thetechnique represented in FIG. 5 will preferably be used, this beingstrictly axisymmetric and the central cable itself then being the sonarsupport cable 805, onto which the transducers will have been threadedand fixed in the correct position. It would optionally be possible tosling the emission array under the sonar.

One of the problems of the invention in the towed mode consists, asindicated above, in maintaining the linearity of the linear array overthe whole of its length, since the hauling system would tend to curveit, at least slightly.

In order that this condition can be best complied with, the inventionproposes, by way of enhancement, to use for example a triangulatedrigging in which the array 303 is simultaneously hauled by its upperpart and by its lower part by two separate cables, this making itpossible to compensate for the greatest action of the drag on the bottomof the array.

With raising the array, so as to be able to wind it easily, the lowercable then has to be detached and wound onto a second reel, the start-upof which will be delayed so as to obtain correct winding of the array.

For placement in the water the reverse operation is carried out.

Another variant consists in using a propelled rigging, in which at thelower end of the linear array 303 is fixed a propeller motor suppliedfrom the boat, by way of the towing cable, with an electrical signalvarying as a function of the speed of the towboat, in such a way as tomaintain the linearity of the array by compensating for the excess dragapplied to the lower end thereof. It is also possible to use severalmotors of this type spread along the array.

As a variant, the invention also proposes, as represented in FIG. 9, touse a towed fish 304 to tow and wind up the linear array 303, this fishbeing similar to that used in the prior art and being dragged by thecable 305 and itself dragging the receiving array 301. According to theinvention, inside it this fish 304 includes a winch 314 to which thelinear emission array 303 is fixed by a stub of cable. To raise theassembly on board the towboat 302, the emission array 303 is firstlywound up on the winch 314 and then the cable 305 is wound up on thetowing winch fixed to the boat. The receiving array 301 is lastly woundonto a separate winch, according to a known technique when using anordinary fish.

Apart from the example embodiment described above in which thetransducers lie inside a flexible jacket and are separated from oneanother, a beneficial variant embodiment consists in using leaktightelementary modules of streamlined shape linked together by cabling whichis itself leaktight. This makes it possible to simplify the embodimentof the array and in this case a beneficial enhancement, to make itpossible to maintain the straight shape of the array in the waterdespite the hauling loads and vibrations, consists in furnishing theends of these separate modules with a bearing face which matches thebearing face of the adjacent module. Under these conditions, in order torigidify the array use will be made of a cable which runs along it andwhich may moreover be one of the assembly cables. This cable will befixed on one side to one of the ends, the lower end for example, of thearray and on the other side to a small winch furnished with a windingmotor. Thus by actuating this winding motor the modules will be broughtcloser together and will lock against one another, thus ensuring therigidity and straight shape of the array.

In a preferred embodiment of the structure of the array, modules such asrepresented in FIG. 10 will be used, comprising a flexural strain gaugeof the IV type with an ovoid cross-section. The rear of this flexuralstrain gauge is extended by a fairing 111 of substantially triangularcross-section, so as to accord the assembly a streamlined cross-sectionin the shape of a symmetric aircraft wing.

The ends of this flexural strain gauge terminate in shackles, of whichone 112 is double and the other 113 is single. Thus one shackle 113 willbe inserted into a shackle 112 of the adjacent module, and the two arefixed together with the aid of a pin 114 which goes through the holesfor assembling the shackles.

The supply signals and control signals are propagated by way ofelectrical cables 115 which enter at one end of the module and leave itat the other end by way of leaktight seal-ways. The whole issufficiently flexible to be able to be wound up on a reel, thedimensions of which are for example of the order of two meters. It isalso possible to secure the assembly with the aid of ties passing rightthrough the modules, here again by way of leaktight seal-ways. Asappropriate, by making suitable bearing surfaces, for example planar, atthe ends of the modules and by according an oblong shape to the holes inthe shackles into which the pins 114 will be inserted, it will bepossible to use a method of rigidification such as described above.

I claim:
 1. Process of acoustic emission for a sonar, comprising thesteps of:towing in water with the aid of a hauling cable, a passivelinear acoustic array; and towing a separate linear acoustic array withsaid hauling cable wherein said separate linear acoustic array is heldsubstantially vertical and emits signals received by said passive linearacoustic array.
 2. Device for implementing an acoustic emission for asonar comprising:a passive linear acoustic array; a separate linearacoustic array for emitting acoustic signals whose echos are received bysaid passive linear acoustic array; a hauling cable for towing saidpassive linear acoustic array and said separate linear acoustic arraywherein said separate linear acoustic array has a weight and shapeenabling it to be towed at a small inclination relative to the verticalwhile the receiving array is towed substantially horizontally.
 3. Devicefor implementing acoustic emission, comprising:a sonar with deployablereceiving array suspended from a helicopter by a cable; and a linearacoustic emission array suspended above or beneath the receiving arraywherein the cable forms an integral part of the linear emission array.4. A process of acoustic emission, comprising the steps of:suspending asonar with deployable receiving array from a helicopter by a cable;suspending from said helicopter, a linear acoustic emission arraywherein said linear acoustic emission array is suspended above orbeneath the receiving array wherein said linear acoustic emission arrayemits acoustic signals whose echos are received by said receiving arrayor in said linear acoustic array which is held substantially vertical.5. Device according to claim 3, characterized in that the linearacoustic emission array is formed by a string of transducers fixed alongat least one cable.
 6. Device according to claim 5, characterized inthat the transducers are of the flexural strain gauge type.
 7. Deviceaccording to claim 5, characterized in that it includes means forsupplying the transducers with variably weighted signals for controllingthe shape of the main emission lobe.
 8. Device according to claim 7,characterized in that it furthermore comprises means for supplying thetransducers with phase-shifted signals making it possible to shift theaim of the emission beam.
 9. Device according to claim 2, characterizedin that it furthermore comprises a fish dragged by the towing cable anditself towing the linear receiving array; this fish including anintegrated winch to which the linear emission array is fastened so as toenable this array to be wound in before the fish and the linearreceiving array are together raised onto the hauling vessel.
 10. Deviceaccording to claim 9, characterized in that the linear emission array isformed by a set of identical modules comprising a transducer of theflexural strain gauge type with substantially ovoid cross-sectionfurnished with a rear fairing of substantially triangular cross-sectionand including, at its two ends, shackles of which one is single whilstthe other is double so as to enable the single shackle of one module tobe inserted into the double shackle of the adjacent module and to jointhem together with a fixing pin.
 11. Device according to claim 10,characterized in that the transducers are articulated together so as toform a winch-wound assembly.
 12. Device according to claim 10,characterized in that the emission array comprises means formechanically interlocking these articulations so as to ensure thestraightness of thearray during emission and so as to release thislocking during the operations of winding and unwinding the array on awinch.